This invention relates to resonators and more particularly to tunable coaxial cavity resonators for high power tubes.
Coaxial output cavity resonators are well known in the state of the art. The purpose of the output cavity of an active device is to transform the load impedance to an impedance suitable for the active device to develop the stated power output. High power tubes have a finite output capacitance. Real power can only be developed in the resistive portion of the tube load impedance; therefore, the output circuit must also resonate the tube output capacitance. The output circuit must be capable of operating with extremely high voltages and currents when handling high power. In a typical arrangement, a transmission line resonate cavity is added to the output of the tube and the RF power is coupled from the resonate cavity by either a coupling capacitor or by a coupling loop. Difficulty occurs when the circuit is operated over a broad range of frequencies. In attempting to maintain the desired power with changing frequencies in the case of the coupling capacitor, the capacitor may be moved too close to the transmission line causing voltage breakdown to occur. Similarly, in the coupling loop case when moving the loop in and out, the loop can be moved too close causing voltage breakdown to occur. A change in capacitor size or loop size or physically moving the coupler to another place in the line to increase power coupling would be time consuming and impractical with frequent frequency changes. Direct tap to the transmission line works but only at one frequency. Movable direct taps can provide tuning over a relatively broad range of frequencies but there are many applications where physically moving the output coupler is not desirable or not possible. The same is true for moving the capacitor or inductor.
Briefly, an improved coaxial cavity resonator for an active device which is tunable over a given band frequencies, is provided by an inner conductor, a coaxial outer conductor, a third intermediate conductor and tuning stubs. The inner conductor and the coaxial outer conductor are coupled to the active device and the third intermediate conductor is spaced coaxially between the inner conductor and the outer conductor with the intermediate conductor beginning at a given distance along the cavity from the active device and extending to that end of the cavity remote from the active device. A fixed position output coupler is connected directly to the third intermediate conductor at the given distance from the active device. The inner and outer conductors are dimensioned and are of the given electrical length from the active device to where the intermediate conductor begins to present an inductive impedance at the inner and outer conductors at the coupling region of the output coupling device. The first of the shorting stubs is positioned between the inner conductor and the intermediate conductor, and the second of the shorting stubs is positioned between the intermediate conductor and the outer conductor. The stubs are positioned such that a capacitive reactance is presented across the inductive impedance to resonate therewith and selectively tune the cavity to frequencies over a given broad band. The relative position of the two coaxial stubs controls the transmission of the tube impedance to that of the load.